The seasonal decline in clutch size in birds can be a response to the environmentallyconditioned decrease in prospects for offspring or a consequence of a lower physical abilityof late-breeding females. To find out which of the explanations apply in Tree SwallowsTachycineta bicolor, we assessed whether replacement clutch size in this species isaffected by an individual female’s ability to lay a certain number of eggs. To do this, wemeasured the decline in clutch size as a function of laying date between first and replacementclutches in individuals that re-nested following natural failure, and compared thiswith the rate of decline in clutch size with laying date for Tree Swallows that laid only asingle clutch in that season. Additionally, we assessed whether the clutch size and therate of its seasonal decline varied across years. We accounted for the truncated andunder-dispersed nature of clutch size data by using a Bayesian approach in the analysis.We found little variation in the rate of clutch size decline across years at our breedingsite. Accounting for this seasonal decline in clutch size, mean clutch size was similarbetween single-time breeding females and those that laid replacement clutches, implyingthat the number of eggs laid on the second attempt by female Tree Swallows is determinedby laying date, rather than by the female’s physical ability to produce a clutch ofa certain size.,